Expedition to Dongsha Atoll

Researchers from Anne Cohen's lab at Woods Hole Oceanographic Institution and the Coastal Dynamics Lab at UC Irvine are traveling to Dongsha Atoll, a remote coral reef ecosystem located south of Taiwan in the South China Sea. We will be on the atoll May 22 - June 26, deploying a range of instruments, collecting seawater and coral samples and making measurements of Net Ecosystem Calcification (NEC). We will be joined by scientists from Academia Sinica and Texas A&M University.

Kristen Davis's lab

Science

Thursday, June 26, 2014

We all had mixed feelings getting on the plane yesterday to leave Dongsha. Fresh fruit (mangos!) and vegetables (anything actually green!) await us back in Taiwan, but we are leaving behind the friends we made on Dongsha over the past month. Excited to begin digging into all the fascinating data that we collected, a bit sad to leave the beautiful coral reefs on Dongsha. Clean showers ahead, leaving behind that strangely gratifying feeling of being covered in salt after a long day working in the ocean.

The plane ride home is also a time to reflect on the amazing success of our expedition. On a reef that is a magnet for tropical storms (including a direct hit form Tropical Storm Hagibis a couple weeks ago!), strong monsoon winds, covered with fishermen, and in most places out of site of any land, we not only deployed a series of very expensive instruments and set up a 4-story scaffolding tower, we actually recovered everything a month later. As Dr. Soong, the head of the research station on Dongsha told us as we boarded the plane, "I never would have believed it in a million years".

My buddy Aloha of the Marine National Park

Of course, our team on Dongsha is only part of the story of that success. We owe huge thanks to so many who helped along the way: Dr. Wong and his group at Academia Sinica, Dr. Soong from National Sun Yat-Sen University, the Dongsha Atoll Marine National Park, our boat captains who put up with all our requests to go everywhere around Dongsha, and all the other researchers on Dongsha who tagged along to help us find temperature loggers and set up our scaffolding tower. Thank you all!

Sunday, June 22, 2014

What’s
a field expedition without the instrument recovery, cleanup, and packing duties?
During the past week, our group has been working meticulously to retrieve our
sea-faring instruments and prepare them to be shipped back home. Kristen, Pat,
and Tom made a heroic effort in diving down to the various mooring locations
and searching for the often obscured and barnacle-laden instruments. To our
luck, most of the equipment
has been recovered. Some of our moorings were in locations susceptible to
strong currents and wave action, and were likely moved and bumped around during
the last few weeks. This made our dive team’s job of finding the moorings more
difficult, but their diligence and experience prevailed.

All the nooks and crannies...
poor Tom!

Although our devices have only been in the ocean for what
seems like a relatively short time span, we still found that upon recovery, our
instruments were covered with a slimy layer of sea gunk: algae, barnacles,
mollusks, and various autotrophic marine organisms that find shiny, white
temperature and pressure loggers attractive. Cleaning off all of this deliciousness
was simply a matter of scrubbing the devices with abrasive pads and water,
though some of us had a much more daunting checklist of items to clean.
Ultimately, everyone pitched in and the work basically took care of itself. Something
we hardly take for granted is the foresight that the engineers who designed
these instruments had when they were making these devices; plastic casings,
titanium hardware, and stainless steel mountings offer fantastic corrosion-resistance.

Cleaning instruments: more fun
than it sounds like.

One
of the more exciting moments of our trip came when we began uploading and
viewing data from the instruments. Seeing firsthand the preliminary recordings
of temperature, pressure, pH, oxygen, and currents, amongst other variables,
that reflect the highly dynamic conditions, perhaps unique to Dongsha atoll and
characteristic of a coral reef exposed to such large internal waves, made
enduring a month of, well…interesting
food worth it.
All of us are eager to begin a more rigorous analysis of our data to better
understand and demonstrate how internal waves affect corals on Dongsha Atoll, however,
we must first be good stewards of our equipment, and finish properly cleaning
and packing them, so as to prolong their useful lives; after all, this stuff ain’t cheap!

Wednesday, June 18, 2014

After a week stuck inside while Tropical Storm Hagibis passed directly over Dongsha, we finally got back to our usual routine of research out on the reef. And when we got back out there, we were relieved to see that the corals are back to normal too.

The same coral colony bleached white on May 26 (top)and recovered from bleaching on June 17 (bottom).

Corals that were bone-white a week ago were again green, a sure-fire sign that the corals are back to hosting photosynthetic symbionts. It appears the bleaching event is over at last.

While this is certainly good news, it doesn't mean all is well. We saw some mortality, so for some corals relief came too late. And the next bleaching event may be even more destructive.

Samples that we collected throughout the bleaching event will help us understand the resiliency of Dongsha corals. Using our coral cores, we can check how frequently bleaching occurs. We will also track how coral energetics (fat reserves, for example) changed as the corals bleached and then recovered. Further, we will see if the coral symbionts after the bleaching event are different than before bleaching. Certain "clades" of symbionts are more heat-tolerant than others, so the corals may have swapped out symbionts to better handle the heat.

Wednesday, June 11, 2014

A tough team waits out a squall on the tower to finishpreparing the RAS to go back in the water.

Enjoying a windy (and wet!) bike ride down the Dongsha Airport Runway

Anyone who does much field work will tell you that for any
field expedition it is important to plan some time for bad weather, or some
“weather days”. Well, we’re using some
of our weather days now! The last few
days have brought higher winds, rough seas, and rain. We are all eager to get back out on the water
to download data, check on instruments, and survey the bleaching, but until the
weather turns around we are stuck in the lab – which leaves some time for
preliminary data analysis, catching up on email, and bike rides down the
runway!

Fresh mango - a.k.a. "heaven"!

During this time w

e were also fortunate to meet some other researchers from
the National Sun Yat-sen University in Kaohsiung, Taiwan, visiting Dongsha
Atoll. We enjoyed learning
from them about their work on the reef and were very grateful for the fresh mango
and bananas they shared with us. Fresh
fruit is a rare treat on Dongsha!

Tuesday, June 10, 2014

drillbit into the skeleton. The black hose is connectedto a spare SCUBA tank that powers the drill.

Our measurements of net ecosystem calcification (NEC) and our observations of coral bleaching, though incredibly valuable, are brief snapshots of Dongsha Atoll. We can also learn a lot about the reef from a longer term perspective. How have calcification rates on Dongsha changed in the past, and why?

Ideally, we could repeat our expedition year after year to see how calcification on the reef changes under different climate conditions. But I do need to write a thesis, so we need a different approach. One alternative is to use the growth histories preserved within the skeletons of massive corals living on Dongsha.

CT scans of corals collected from Green

Island, southeast of Taiwan. The

alternating light and dark bands are

high and low density bands, respectively.

Each year, one pair of high and low

density bands form, providing a timescale

to interpret past growth rates. Counting

back the bands, and measuring the distance

between them, tells us how growth rates

have changed over the past several decades.

How do corals tell us about the past? Corals are constantly building their skeletons, the colony grows larger year after year. Yet corals respond to their environment, building skeleton of different density during different seasons of the year. This seasonal rhythm of skeletal density creates an intrinsic timescale within the skeleton, analogous to tree rings. As the colony continues to grow outward, the growth history of past decades, and sometimes even centuries, is preserved within the interior of the colony.

To access this information, we use underwater pneumatic drills to collect cores of skeleton from living coral colonies. Don't worry, we patch up the holes with cement and underwater epoxy and the coral grows over our plug in about a year. Back in Woods Hole, we send our cores through a CT scanner, which gives us a 3D map of skeletal density. Using software developed in our lab, we use the annual density banding to reconstruct coral growth rates in the past.

This information tells us how coral calcification on Dongsha changed during especially warm or cool years, or whether calcification is increasing or decreasing with time. Corals even form anomalous bands when they recover from bleaching, so we can tell when these corals have bleached in the past. How unusual is the bleaching event that we are seeing right now? Our cores will help answer that question.

Coral reefs are amazingly dynamic ecosystems. Everywhere in the ocean, organisms are constantly changing the chemistry of the seawater around them. Seasonal variability often dominates in the open ocean, where the composition of the upper 100 meters changes relatively slowly as plankton photosynthesize and respire. Changes are "slow" because there is a lot of water, and plankton are very small.

On a coral reef flat, the table is turned. Here, there is much less water (1-2 meters) and reefs are teeming with life. The timescale of change shrinks. Seawater chemistry can change dramatically over the course of a day. Tracking these changes tells us a lot about the reef, such as the rates of net calcification (construction of calcium carbonate minus dissolution) and net production (photosynthesis minus respiration).

The "robot" of our sampling team, the RAS collects

seawater samples on the shallow reef flat. The smaller

instrument next to the RAS is measuring the pH of the

seawater.

But tracking changes in seawater in such a dynamic system requires an intense sampling effort. On Dongsha, we deployed the RAS (see June 1 post) to automatically collect samples every 2 hours on the reef. In addition, the research vessel Ocean Researcher 3 (OR3) steamed from Taiwan to Dongsha to collect samples just offshore of the reef every few hours for several days. During the day, we bring two more boats to the reef: the larger Atoll-2 is stationed just inside of the lagoon while the smaller Atoll-1 travels over the shallow reef to sample between the RAS and OR3. It was at these times that our months of planning truly felt worthwhile. A whole transect of sampling teams (including 1 "robot") stretched across the reef, perfectly synchronized. And at the same time, the DTS and offshore mooring (posts on June 3 & 7) are characterizing the physical processes around the atoll. Together, the samples that we collect will paint a unique picture of the dynamic processes occurring on Dongsha Atoll.

The
immense spatial nature of internal waves, stretching thousands of feet across
different density layers below the ocean's surface, leads to their important role in the
mixing of ocean waters. Here at Dongsha in the South China Sea, observing the
water column as it flows from the ocean, over the reef, and into the lagoon, requires
a baseline measurement of temperature and currents offshore of the reef. A few
days ago we successfully installed ‘E1’, our dense 16 meter mooring that holds
7 temperature loggers spaced evenly between a temperature and pressure sensor
at the top, and a MicroCAT (temperature + pressure + oxygen sensor) at the
bottom, in the deep offshore section of the reef. Each temperature logger is 2
m apart, allowing us to observe the thermal stratification of the water column
before it enters the lagoon.

4 large buckets filled with concrete helped
weigh down the 'E1' mooring.

We were also fortunate enough to
have Tom and Kristen scuba down a platform that holds the AWAC, an acoustic
surface tracking instrument that profiles currents and wave direction, and a pH
meter to a depth of ~20 m. We already installed a similar platform in the
shallower lagoonal side of the reef, which contains an ADCP (acoustic Doppler
current profiler) and another MicroCAT. This hard work marks a significant
portion of our efforts to characterize flow across the reef,
and will make for some interesting data!